1. Technical Field
This invention relates to regulated self-bias circuits connected to circuits including a High Electron Mobility Transistor (HEMT) and, more particularly, to monolithic regulated self-biased circuits for low noise HEMT amplifiers.
2. Discussion
As gate geometries for low-noise Gallium Arsenide (GaAs) Field-Effect Transistors approach a lower size limit (approximately 0.25 micrometers), performance also approaches a limit. High Electron Mobility Transistors (HEMTs) have superior performance and can be fabricated on GaAs. A carrier transport mechanism of the HEMT resembles transport within undoped GaAs which has little impurity scattering. The structure of the HEMT is similar to a GaAs Metal Semi-conductor Field-Effect Transistor (MESFET), utilizing a hetero junction between GaAs and Aluminum Gallium Arsenide (AlGaAs). The HEMT have almost twice the carrier velocity and mobility of a MESFET, resulting in a higher cutoff frequency and lower noise figure.
Most communication systems utilize receivers requiring low noise amplifiers. HEMT have low noise characteristics and have been used in the communication receivers. However, threshold voltages of the HEMT are difficult to control and vary due to an inherent physical nature of the HEMT device process fabrication and material growth. In addition to material and process variations, the HEMT threshold voltage will also vary with temperature and age. Conventional receivers using HEMT devices currently require an off-chip threshold voltage regulator tolerant of the threshold voltage variations which adversely impacts chip size and cost.
Combining a self-bias regulation circuit with a HEMT amplifier or other types of HEMT integrated circuit functions in a microwave monolithic integrated circuit (MMIC) is desirable. A common goal for any monolithic circuit is to use design and fabrication techniques to reduce chip size, increase ease of implementation, and reduce costs in high volume production. These design and fabrication techniques should not compromise performance, and if possible, should improve performance.